The files go in "GameData", not in "base" with exception if there's a base folder within the archive the contents of that folder should be placed in base. It means you didn't install it correctly
Enabling cvars that are marked as cheats without making a change to the mod itself requires a hack. And since the ja+ mod is defunct that's not possible
They are updating it for df2mod mostly with plans to be merged back at some point. Presently the existing rend 2 doesn't work with the current builds of OpenJK though.
Don't use the version in the tutorial on jkhub. Get the latest version of cmake from the cmake website. Also make sure you have the C++ component installed with Visual Studio, if that isn't installed it won't work or detect a compiler.
That doesn't really make any sense because neither the original game nor OpenJK were ever compiled against Visual C++ 2005. The former for obvious reasons, and the latter because we had a minimum of 2010 as oldest originally. Coincidence perhaps?
OpenJK does not use jaconfig. For MP it's just "openjk.cfg", for SP: "openjk_sp.cfg" afaik. Also, these files are located in your Documents folder. You'll want to look for "My Games" and "OpenJK" then the base folder.